The present invention relates to a structure of a photoconductive film which is used for a target of a photoconductive image pickup tube and, more particularly, to a photoconductive film which can decrease a drift in sensitivity just after the image pickup tube is switched on among the photo response properties of the rectifying contact type photoconductive film.
As is well known, amorphous Se exhibits a photoconductivity and a photoconductive film of the rectifying contact type can be produced by combining this amorphous Se with a signal electrode of an n-type conductivity. In this case, since Se doesn't have a sensitivity to the long wavelength light, a method whereby Te is added into a part of the Se film is adopted to improve the above-mentioned sensitivity (U.S. Pat. Nos. 3,890,525 and 4,040,985 and Japanese Pat. No. 1083551 (Publication No. Sho. 56-6628)).
On one hand, to decrease the after image to the strong light, a method whereby GaF.sub.3, MoO.sub.3, In.sub.2 O.sub.3, etc. are added into a part of the Se film is adopted (U.S. Pat. No. 4,463,279). FIG. 1 shows a principal structure diagram of the target according to a conventional technology. In this diagram, a reference numeral 1 denotes a transparent substrate; 2 is a transparent electrode; 3 a photosensitizing part of p-type photoconductor; 4 a p-type photoconductive film serving as a layer to reduce the storage capacitance of the target; and 5 an auxiliary layer for assisting the landing of an electron beam. The photo-sensitizing part 3 consists of Se, As, Te, and GaF.sub.3 ; the p-type photoconductive film 4 consists of Se and As; and the beam landing aiding layer 5 consists of Sb.sub.2 S.sub.3. FIG. 2 shows an example of component distribution in the direction of the film thickness of the photo-sensitizing part 3 in FIG. 1. In this example, Te for increasing the sensitivity doesn't exist at all at the position (position indicated by a) corresponding to the interface with the transparent electrode 2 where the film thickness is zero. The concentration of Te rapidly increases from the position of the film thickness of 500 .ANG. and Te is added into the region (portion b) over 1000 .ANG. thick to the position of the film thickness of 1500 .ANG.. Substance As added into the portions a and b serves to increase the thermal stability of Se. Substances As and GaF.sub.3 are added into the portion indicated by c, in which it is considered that As serves to form a deep trap level to capture electrons in Se and GaF.sub.3 serves to form negative space charges by capturing the electrons in Se. The portion c allows the after image for the strong light to be decreased and simultaneously permits the sensitizing effect to be increased. The concentration of As decreases over the film thickness of 1000 .ANG. at a uniform gradient. The concentration distribution of GaF.sub.3 is uniform over the film thickness of 1000 .ANG.. The target having such a structure can attain the object of increasing the sensitivity to the long wavelength light and to decrease the after image to the strong light, while the properties such as the lag, resolution and the like which are ordinarily required as an image pickup tube are good. However, this target has a drawback such that if the film thickness of the region where the negative space charges are formed is thick, time drift in sensitivity just after the image pickup tube is switched on is large.